Ethylene resin foamed products have excellent flexibility and heat insulating properties, so that they have been hitherto applied to various uses as cushioning materials or heat insulating materials. The type of a polyethylene resin is selected according to the use purpose of the resulting foamed product. For example, when the foamed product requires flexibility, low-density polyethylene is used, and when the foamed product requires toughness, linear low-density polyethylene or a resin composition consisting of low-density polyethylene and linear low-density polyethylene is used (see Japanese Patent Publication No. 57334/1986).
The linear low-density polyethylene is a copolymer of ethylene and an .alpha.-olefin, and in order to enhance flexibility, a resin produced using an increased amount of an .alpha.-olefin that is a comonomer component is used. In such a resin, however, a low-molecular weight polymer portion in which the comonomer component is introduced into the molecular chains and a high-molecular weight polymer portion in which the comonomer component is rarely introduced are present separately from each other, so that a wide scatter of melt viscosity value occurs in the resin to thereby lower foamability of the resin. For example, the expansion ratio does not increase, or even if a foamed product having a satisfactory expansion ratio is obtained, the product has a problem in appearance such as protrusions or depressions, or the product contains both of extremely large cells and small cells and is broken from the ununiformly foamed portion during the forming process.
As tape substrates or sheet substrates used for compress materials or anti-inflammatory analgesic plasters, foamed products of small thickness are used instead of cloths or flexible synthetic resins, and such tape substrates or sheet substrates are required to have flexibility and toughness (favorable extensibility and tensile strength). As the tape substrates or the sheet substrates of this kind, crosslinked foamed products comprising a resin composition consisting of an ethylene/vinyl acetate copolymer and linear low-density polyethylene have been proposed (Japanese Patent Publication No. 33387/1990). The crosslinked foamed products as tape substrates, however, have bad balance between flexibility and toughness. If the amount of the ethylene/vinyl acetate copolymer is increased in order to obtain products of more flexibility, the tensile strength of the products is lowered.
The conventional polyethylene resins are markedly lowered in the viscosity when they are exposed to high temperatures or subjected to melting, so that crosslinking is generally made to ensure viscoelasticity of such a level as required in the foaming process and to retain the produced cells. For crosslinking the polyethylene resins, various methods such as an irradiation crosslinking method using ionizing radiation, a peroxide crosslinking method in which resin radicals are produced by a peroxide to perform crosslinking, crosslinking method adding a multifunctional monomer in the produced radicals and a crosslinking method by means of silanol condensation (silane crosslinking method) have been industrially carried out. Uncrosslinked polyethylene resin foamed products are industrially produced by dispersing or dissolving in a resin a substance that is gasified at ordinary temperature and atmospheric pressure or upon heating, such as carbonic acid gas, methanol, water or flon, and subjecting the dispersion or the solution to extrusion foaming or batch foaming.
The conventional resins, however, are difficult to control degree of crosslinking, and it is not easy to produce crosslinked foamed products of certain qualities. Further, when the conventional polyethylene resins are not crosslinked, the viscosity of the resins is markedly lowered under the foaming conditions as described above, and hence it is very difficult to obtain uncrosslinked foamed products. In particular, it is difficult to produce conventional uncrosslinked polyethylene foamed products as continuous sheet products, although it is possible to produce them as small-area products such as rod or tubular products.
The present invention is intended to solve such problems associated with the prior art as described above, and it is an object of the invention to provide ethylene resin foamed products containing uniform cells and having excellent appearance, toughness and flexibility.